I understand that you are trying to use a variable created in workspace as a parameter to define the dimension of the data variable. You can use a variable from workspace as a parameter to the bus object, but the variable should be already loaded in the workspace or defined in the callbacks. You can do that by navigating as follows
Modelling-->Model Settings(drop down)-->Model Properties-->callbacks
In the callbacks you can define and declare the variable, click 'apply' and 'ok'
